This invention relates generally to the treatment of the surfaces of devices, especially semiconductor wafers and other electronic or electro-optical devices, at various stages of production. It relates more particularly to novel cleaning, chemical treatment and drying processes in which, instead of a condensed phase medium (liquid), a foam is used as a medium for the various operations such as cleaning, etching, neutralization and drying.
Semiconductor cleaning, chemical treatment and drying technology is well developed. Examples of known processes are those described in U.S. Pat. No. 4,781,764, dated Nov. 1, 1988, U.S. Pat. No. 4,911,761, dated Mar. 27, 1990, U.S. Pat. No. 5,271,774, dated Dec. 21, 1993 U.S. Pat. No. 5,656,097, dated Aug. 12, 1997 and U.S. Pat. No. 5,571,337, dated Nov. 5, 1996. However, cleaning, chemical treatment and drying of semiconductors is very expensive. Moreover, performance requirements will soon exceed the present and expected capabilities of current cleaning techniques.
Current processes for the cleaning and treatment of semiconductor wafers and other electronic devices have several serious drawbacks from the standpoint of cost, safety and effectiveness.
High purity deionized water is typically used as a solvent. However, achieving the necessary high purity levels is very expensive. Indeed, all phases of the cleaning operation, including purchasing, transportation, storage, internal distribution, consumption, and disposal, are expensive.
Most of the substances used in the cleaning and chemical treatment processes, such as fluorides, solvents, acids, heavy metals, oxidizers, etc., are toxic, flammable, or otherwise hazardous or obnoxious.
Chemical treatment and cleaning operations are also major sources of chemical contamination of the final product. Such contamination results from errant surface reactants, and physical contamination by undesired, very small solid particles. These very low levels of contaminants are delivered to the product, in part from the chemical treating and cleaning materials themselves, even though they are ultrapure. They are also delivered to the product from fittings, piping, tanks, valves, and other components of storage and delivery systems.
Contaminants on semiconductor wafer surfaces exist as films, discrete particles or groups of particles and adsorbed gases. Surface films and particles can be classified as molecular compounds, ionic materials and atomic species. Molecular compounds are mostly particles or films of condensed organic vapors from lubricants, greases, photo resists, solvent residues, organic components from deionized water or plastic storage containers, and metal oxides or hydroxides. Ionic materials comprise cations and anions, mostly from inorganic compounds that may be physically adsorbed or chemically bonded, such as ions of sodium, fluorine and chlorine. Atomic or elemental species comprise metals, such as gold and copper, which may be chemically bonded to the semiconductor surface, or they may consist of silicon particles or metal debris from equipment.
Semiconductor devices, especially dense integrated circuits, are vulnerable to all of these contamination sources. The sensitivity is due to the small feature sizes and the thinness of the deposited layers on the wafer surface. These dimensions are in the submicron range. The small physical dimensions of the devices make them very vulnerable to particulate contamination in the air, from workers, generated by the equipment, and present in processing chemicals. As the feature size and films become smaller, the allowable particle size must be controlled to smaller dimensions. In general, the particle size should be 10 times smaller than the minimum feature size. Currently, the minimum feature size for commonly available semiconductor chips is 0.25.mu., therefore suggesting particle control to 0.025.mu..
Conventional cleaning technologies, utilizing condensed phase solutions, when properly applied, remove a majority of the contaminants generated during the chemical processing of the semiconductor wafers. Liquid systems currently in use can delivery satisfactory results, and acceptable product can be produced. However, the current trend is to require the chemical and equipment suppliers to provide increasingly clean performance. Equipment and chemical suppliers are facing tremendous performance challenges as the feature size decreases. At the same time, semiconductor manufacturers do not want their costs to increase.
Another problem addressed by this invention is the drying of surfaces in the production of semiconductor wafers and similar devices.
Semiconductor wafers are not manufactured in a continuous process. Since there are many semiconductor wafer configurations, batches of wafers are processed through certain steps, and then stored. Later the batches are subjected to additional processing steps, and again stored. The processing and storage sequence may be repeated several times before processing is completed.
In general, at the end of each process sequence, the semiconductor wafers are dried, often even when the next step will proceed almost immediately. Wafers can be transported from one process sequence to the next only after they have been dried, and they can only be stored safely when they are dry. Therefore, the drying process is carried out frequently in the processing of a given wafer, and is very important.
Recently, isopropyl alcohol has become a preferred drying solvent. A variety of processes have been developed and commercialized using isopropyl alcohol either hot or cold, and as a vapor, a liquid or a combination of vapor and liquid. Semiconductor wafer producers have been moving toward reduced isopropyl alcohol usage because of its cost, fire hazards, disposal problems, and VOC (volatile organic compound) emissions.
U.S. Pat. No. 4,911,761, dated Mar. 27, 1990, describes semiconductor wafer processing in which various fluids passed over wafers in fixed positions. The drying sub-system utilizes superheated isopropyl alcohol vapor generated in a distillation apparatus.
U.S. Pat. No. 5,271,774, dated Dec. 21, 1993 describes a technique for removing water from a semiconductor wafer using low levels of solvents such as isopropyl alcohol, applied as a vapor, to reduce the surface tension of a film of liquid on the substrate, and thereby reduce the quantity of material remaining on the surface of the substrate. A centrifuge is used to facilitate the removal of the liquid film.
U.S. Pat. No. 5,571,337 describes another technique for drying semiconductor wafers, utilizing a trace amount of a polar organic compound in a carrier gas composed of oxygen, nitrogen, argon, or mixtures. This patent describes the drying of wafers without the use of isopropyl alcohol, using only warm nitrogen gas. Thus, the industry has proceeded from the use of large quantities of isopropyl alcohol, to minimal quantities of isopropyl alcohol, and then to processes which use no isopropyl alcohol at all.
The principal objects of this invention are to increase the effectiveness of chemical treatment, cleaning and drying operations, and to reduce the cost of such operations. Further objects of the invention are to improve the safety of the chemical treatment, cleaning and drying operations and to reduce the discharge of hazardous or obnoxious substances from the treating and cleaning operations.